Effects of spay or neuter on behavior & performance

Effects of Gonadectomy on Health, Behavior and Performance of Petshttp://www.vivo.colostate.edu/hbooks/pathphys/reprod/petpop/proscons.html
Are there reasons to recommend castrating or spaying pets other than to feel good about not contributing to the pet overpopulation problem and avoiding the hassle of having a litter in your home? Like any other medical procedure, gonadectomy decreases the risks of some conditions while increasing those of others. One thing that seems clear is that gonadectomized dogs and cats are less likely to be relinquished by their owners to humane organizations than sexually-intact pets, largely due to beneficial effects on behavior; in other words, animals that are gonadectomized, in general, make more desirable family members.
Effects on Behavior

Gonadectomy effectively eliminates reproductive cycles and estrous behavior in females, which is probably the foremost benefit sought by pet owners that spay their bitch or queen. Having your pet spayed will also eliminate your home as a regular neighborhood attraction for roaming males looking for a good time. To convince yourself that these are substantive benefits, volunteer to take care of a Siamese queen while she is in heat.

It must be admitted that males are a more common source of objectional behavior than females. Such behavior is a common cause for owners to relinquish their pets. Several studies have addressed the question of how castration affects behavior in dogs and cats. To summarize:

Dogs

* Roaming, urine marking in the house, and mounting people, inanimate objects and other animals: A majority of dogs (50-70%) show very significant decreases in these behaviors following castration. Importantly, the age of the dog at the time of castration had little influence on outcome. In other words, the likelihood that castration prior to puberty will prevent these behaviors from developing is essentially that same as after castration in adulthood.
* Aggression: Castration reduces, but does not eliminate, aggression toward other dogs in the family and toward family members. In some dogs, there is also a modest reduction in aggression toward unfamiliar dogs. However, there seems to be no effect of castration on aggressive behavior toward intruders or unfamiliar people.
* Barking, hunting behavior, affection and playfulness: These behaviors, which typically are not different between males and females, appear not to be influenced by castration.

Cats

* Urine spraying: Castration prior to puberty usually prevents this behavior from developing, and most toms castrated after puberty show a rapid decline in this behavior.
* Aggression: Fighting with other males and roaming behavior decline significantly in most castrated cats. The rapidity of this effect varies among males. One consequence of this effect is that cat-bite abscesses are typically much less of a problem in castrated than intact toms.

Effects on Incidence of Disease

Removal of the gonads eliminates or decreases the incidence of several diseases of reproductive and non-reproductive systems:

* Testicular and ovarian disease, including neoplasia, is eliminated. Cancers of the gonads are significant causes of morbidity, especially in dogs.
* Pregnancy-associated disease is eliminated. Examples include uterine torsion or rupture, dystocia, post-partum metritis or hemorrhage.
* Uterine disease, the most important of which is pyometra, is prevented because this organ is also removed as part of a “spay”. For this reason, there seems to be absolutely no justification for performing a simple ovariectomy in dogs and cats.
* There is a clear reduction in incidence of mammary tumors in both dogs and cats. This is a particularly important benefit in dogs, where roughly half of the cancers that develop in intact bitches are mammary tumors. Gonadectomy reduces the risk of developing such tumors 3 to 7-fold, with the greatest reduction in risk seen in those that are spayed at the youngest age.
* Benign prostatic hypertrophy in dogs is effectively prevented as well as cured by castration. This disease, which is very common in older dogs, is typically androgen-dependent and castration removes the major source of androgen. In contrast, castration appears to have no protective or curative effect on prostatic cancer in dogs, and appears to increase risk modestly.

Gonadectomized dogs and cats do face an increased risk for development of certain disorders, sometimes apparently in a breed-specific manner. In dogs, gonadectomy has been associated with increased risk of certain cancers (e.g. transitional cell carcinoma of the bladder) and, in females, estrogen-responsive urinary incontinence. Incontinence is manifest as dribbling of urine, either during sleep or at times of excitement; a majority of such cases are readily treated with estrogen replacement therapy. A related estrogen-responsive problem observed in some spayed bitches is atrophic vaginitis.
Development of Obesity and Lethargy

A concern commonly expressed by owners is that spaying or castrating their pet will cause it to become “fat and lazy”, and indeed, gonadectomy does appear to be a risk factor for devleopment of obesity. Gonadal hormones undoubtedly have effects on energy metabolism, but are not generally considered to be major players in control of food intake and body weight. However, intact cats of both sexes have been shown to have higher metabolic rates than gonadectomized cohorts, and ovariohysterectomized bitches fed free-choice showed higher food intake than intact control bitches.

Relatively few controlled studies have been conducted to assess the effects of gonadectomy on obesity and activity in pet animals. In one study, 44 working German Shepard dogs were either ovariectomized, ovariectomized and given ovarian autographs, or left intact. During the following year, there were no differences in body mass or work performance among the dogs in these groups. In another study, sedentary beagles that were ovariectomized gained a small amount of weight relative to intact control beagles.

Sled dogs provide another indication that neutering has little effect on development of lethargy or obesity. Many of the males and females that run in races such as the Ididarod are castrated or spayed, and it is commonly recommended that neutering be performed if breeding the animal is not intended.
It is clear that additional hard data are required to make a solid judgement on the effect of gonadectomy on obesity and activity level. It appears that spaying bitches has little if any effect on subsequent weight gain or activity level if they receive regular exercise. It appears that many of the antecdotal reports of weight gain following gonadecomy are likely associated with normal aging.